The development of excitability was studied in nerve and muscle cells in culture by measuring neurotoxin binding, 22Na-uptake and electro-physiological parameters of the cells. In embryonic muscle and nerve cells in the normal culture medium, 3H-saxitoxin binding, batrachotoxin-stimulated 22Na-uptake and electrical excitability appeared gradually, reaching maximal values by about 8 days in culture. In the presence of scorpion toxin in the medium, 125I-scorpion toxin binding, batrachotoxin + scorpion toxin stimulated 22Na-uptake and excitability appeared sooner and reached maximal values by about 4 days in culture. These data suggest that excitable site proteins are incorporated into the cell membrane in an inactive form and they undergo a post-translational modification which render them active. A monoclonal antibody is currently being used to purify the excitable site protein. Using squid giant axons, the biochemical processes underlying the deteriorative action of a rise in the internal Ca-ion concentration on excitable membrane sites was examined. It is shown that activation of an endogenous protease by Ca-ion is one of the major factors responsible for suppression of excitability.